1. Field of the Invention
The present invention relates to an engine provided with a lubricating structure, and more specifically, to a single cylinder engine applied to, for example, a motorcycle, provided with an improved lubrication structure.
2. The Related Art
In a single cylinder engine is provided with a crankcase in which a crankshaft extends and with a lubrication structure, so-called forced lubrication-type engine that supplies oil to a surface to be lubricated using an oil pump. The oil, after lubricating a piston of a cylinder and the crankshaft, falls by gravity, and accumulates in an oil reservoir provided below the crankcase of the engine. The oil that has accumulated in the oil reservoir is again pumped by the oil pump and supplied to each of components or parts of the engine.
In such a lubricating structure, the oil after lubricating the piston or the crankshaft passes between a wall that partitions a crank chamber and the crankshaft when falling toward the oil reservoir. At this time, if such oil is not efficiently discharged, the oil stores or remains between the crankshaft and the crank chamber, which causes rotational resistance of the crankshaft and reduces output of the engine. In addition, if the crankshaft rotating at high speed stirs the oil, air may be mixed into the oil, causing oil film shortage in an area to be lubricated, which may cause wear or seizure, thus being inconvenient.
In order to solve these inconveniences mentioned above, there is provided an engine having a lubricating structure achieving an increased discharge performance of oil from a crank chamber (for example, refer to Patent Document 1: Japanese Patent Laid-Open Publication No. 2000-282826). In the lubricating structure described in the Patent Document 1, a space between an inner surface of a crankcase and an outer periphery of a crank web is narrowed on a downstream side of an oil discharge port in a rotational direction of a crankshaft to thereby limit an amount of oil flowing downstream in the rotational direction through the oil discharge port, and hence, increase a discharge performance of oil from a crank chamber.
However, in the lubricating structure of the engine described above, the narrow space is provided between the inner surface of the crankcase and the outer periphery of the crank web on the downstream side in the rotational direction, and thus, the rotational resistance of the crankshaft is further increased by drag resistance. Because of this reason, in application of the lubricating structure described above, it is difficult to maximize output of an engine.
In order to increase discharge performance of oil while reducing the drag resistance of oil in the lubricating structure described above, it is conceivable that a radial space between an inner surface of a crankcase and an outer periphery of a crank web is increased.
In such structure, it is particularly necessary to provide a sufficiently wide radial space between the inner surface of the crankcase and the outer periphery of the crank web in order to ensure an oil discharge path on an upstream side in a rotational direction. If the oil discharge path is to be thus ensured in a radial direction of the crank web, the crankcase has to be significantly widened in the radial direction of the crank web, which undesirably increases a size of an engine.